Lithium ion secondary batteries, a typical example of non-aqueous electrolyte secondary batteries, have high electromotive force and high energy density. Thus, demand is growing for lithium ion secondary batteries, as a main power source for mobile telecommunication devices and mobile electronic devices. A majority of lithium ion secondary batteries currently on the market include a lithium composite oxide containing cobalt as its main component (for example, LixCoO2 (x changes based on charging and discharging of the battery)) as the positive electrode active material. However, cost reduction is difficult because of the high price of cobalt compound used as the raw material in lithium composite oxide containing cobalt as a main component.
Therefore, in view of cost reduction, there have been researches and developments for an alternative to lithium composite oxide containing cobalt as a main component. Particularly, active researches have been carried out for lithium composite oxide containing nickel as a main component (for example, LixNiO2 (x changes based on charging and discharging of the battery)).
In addition to cost reduction, it is important to increase reliability of lithium ion secondary batteries. The lithium composite oxide including Co or Ni produces high valence, highly reactive Co4+ or Ni4+ upon charging. This accelerates electrolyte decomposition reaction involving lithium composite oxide under high temperature environment. As a result, gas generation occurs, and it becomes difficult to curb heat generation at the time of an internal short-circuit.
In charged batteries, LixNiO2 is more reactive than LixCoO2. Thus, curbing of the electrolyte decomposition reaction has been examined. For example, there has been proposed that nickel oxide with the nickel oxidation number of 3 or less and not including lithium in its crystal is added to the lithium composite oxide containing nickel as its main component (Patent Document 1). Additionally, there has been proposed that a coating comprising a specific metal oxide is formed on the positive electrode active material surface (Patent Documents 2 to 5). Further, there have been proposed that the positive electrode active material surface is reduced (Patent Document 6). For example, there has been proposed that the positive electrode active material surface is reduced with gas and the positive electrode active material is mixed with carbon and then baked.
Patent Document 1:
Japanese Laid-Open Patent Publication No. Hei 7-134985
Patent Document 2:
Japanese Laid-Open Patent Publication No. Hei 8-236114
Patent Document 3:
Japanese Laid-Open Patent Publication No. 2003-123750
Patent Document 4:
Japanese Laid-Open Patent Publication No. Hei 11-16566
Patent Document 5:
Japanese Laid-Open Patent Publication No. 2001-256979
Patent Document 6:
Japanese Laid-Open Patent Publication No. Hei 10-199530